If you go for Vista, don't forget you need Ultimate or Enterprise, as licensing prevents you from using Home Basic or Premium.
I, for one, am not convinced this is strictly true. I am not a lawyer either, though!
Looking at the EULA for Home editions, it states that "You may not use the software installed on the licensed device within a virtual (or otherwise emulated) hardware system."
So pop into PC World/elsewhere, buy a retail copy of Home Basic. At this point there is
no licensed device yet. Installation can proceed (in my untrained opinion).
The EULA specifically prevents you using the license key associated with your OEM installed desktop/laptop within a virtual machine as well. This is perfectly sensible as you'd essentially then have two installations having paid one OEM license.
Spot the difference with Business and Ultimate editions, where the same license key can be used on the OEM hardware, and in a single virtual machine who's image is on that same device.
I've searched for explicit explanations from Microsoft and found nothing specifically targetting this precise scenario. It would be expected that Microsoft would not wish make public that this scenario is explicitly allowed, hence why I can't find further commentary from Parallels on this either I suspect.
If you can post links to authoritative statements from Microsoft I'd like to read them.